This invention relates to an FM stereo generator for use as a modulator such as in a test instrument or the like. More particularly, the present invention relates to such a generator wherein the phase between the principal subcarriers is maintained relatively constant and wherein the relative amplitudes of the modulating signals are controlled.
In the United States, FM broadcasting is known and has been widely used. More recently, FM broadcasting has adopted stereophonic broadcasting where both left and right-hand signals are transmitted whereby the left and right-hand signals may be demodulated at the receivers to provide a stereophonic effect. The standards for such stereophonic broadcasting have been set and adopted by the government. Stereophonic FM broadcasting is basically a sum, L+R, and difference, L-R, method. The sum or L+R signal is transmitted on the main channel as normal audio modulation thereby providing an aurally-balanced program for the monophonic listener. The difference or L-R signal is transmitted as a suppressed carrier, amplitude modulated 38 KHz stereophonic subcarrier which frequency modulates the radiated carrier simultaneously with the main channel modulation. Because of the interweaving property of a difference or L-R stereophonic subcarrier signal and a sum or L+R audio signal, it is possible to fully modulate the FM transmitter with the main channel modulation and to insert the stereophonic subcarrier without having to reduce the modulation for either the sum or difference signal.
In accordance with present standards, a maximum modulation of 90 percent is allocated to both the sum and difference signals as applied to the propagated or radiated carrier. The remaining 10 percent of the maximum (100%) modulation allowable is allocated to a pilot subcarrier having a frequency one-half of the stereophonic subcarrier or 19 KHz. The 19 KHz pilot subcarrier frequency is required to be maintained within .+-.2 Hz and the limits of percent modulation of the radiated carrier allocated to the pilot subcarrier is 8 to 10 percent.
Current broadcasting standards also impose constraints on the phase relationship between the pilot subcarrier and the stereophonic subcarrier so that the left and right stereophonic channels are reproduced consistently and at the correct outputs. Further, when the radiated carrier is being deviated upwardly in frequency by the stereophonic subcarrier, a simultaneous positive slope of the pilot subcarrier modulation must also be maintained. The amplitude matching of the main and subchannel signals is required to be within .+-.3.5 percent of one another and the phase difference between the main and sub-channel signals is required to be within 3 degrees at all modulating frequencies between 50 Hz and 15 KHz. The required 75 millisecond pre-emphasis characteristic is applied to both the main and subchannel signals.
In the prior art, synthesizing circuits have been utilized to provide the main and subchannel signals for FM broadcasting. These include both direct and indirect synthesis configurations. In direct synthesis, a large number of separate oscillators are utilized and very sharp filters are further utilized to select the desired signals to add them for application to the FM broadcasting circuit. In the indirect method, the synthesis is provided utilizing phase-locked loops which have the disadvantage in that phase noise is undesirably generated.
These and other disadvantages are overcome by the present invention wherein an FM stereo generator is provided wherein the required phase and amplitude control characteristics are provided to meet government standards without the attendant disadvantages of the prior art.